These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Base Breaker: As with many MMO's and games in general, nearly every update that changes a significant portion of the game causes uproar mixed with approval.
You either think that soaring energy prices are isolating the Free-To-Play base and it needs to stabilize cheaply, or you'll accept that the crown prices are going to increase and that the developers deserve to get paid.
You're assuming the developers get paid more when crown prices for energy are higher. But since all energy enters the game through being bought with real money it makes absolutely no difference to them who buys it or how many crowns it goes for in game money. For every 750 energy consumed they get three dollars, regardless of whether it was auctioned for 4000 crowns or 8000. There's no reason for them to put in a cap, but no reason not to. As long as demand for energy stays the same it makes no difference to their earnings at all.
They seem to actually be experts in manipulating the CE prices. When the prices are getting too high, they tend to release a new item that's only available for a large sum of crowns, causing a high demand for a fast crown injection.
When Three Rings reworked the entire game to remove Mist Energy, the forums basically exploded.
Big Iron never has more than two minions active at once (compare to other bosses who can have from four to eight respawning minions at once), and only has two attacks: A Beam Spam that takes several seconds to charge and blatantly telegraphs where it'll hit, and a tentacle attack that's simple to dodge so long as you keep moving. The main issue for the fight is waiting for Big Iron's shield to go down.
Maulos, meanwhile, is basically a Trojan King Mook who only has the Trojan's overhead swing attack (other Trojans also have a dash attack and an AoE charge attack), it takes a long time for him to wind up the attack, and unlike many other bosses, his minions do not respawn.
Breather Level: Treasure Vaults. No enemies except for a smattering of Kleptolisks, pleasant scenery, treasure boxes and crowns everywhere. As a bonus, getting a Treasure Vault level usually means not getting an Arena or worse, a Graveyard.
It's worth noting that, while Treasure Vaults are certainly the least dangerous of those three level types, they're also typically the least profitable. Catchy music, though...
Colbert Bump: When Team Fortress 2 updated with a patch releasing an item related to Spiral Knights (which had been intergrated with Steam), there was an influx of new users, some enjoying the game, others asking how to get the new hat.
Your mileage won't vary much, though, as this was remembered by the free-to-play knights with starry eyes as the time when Crystal Energy was at an all-time low. You could buy it for a mere 3000 crowns, compared to, as of late 2012, nearly 9000 crowns.
Vog Cub or Skolver, Owlite or Thorn shield or Crest of Almire, Gran Faust/Divine Avenger, Barbarous Thorn Blade/Final Flourish, Glacius, Polaris, Blitz Needle, Krogmo trinkets (especially heart ones). There is some variation, but expect to see those a lot.
Wolver sets! People love 'em.
Gets to the point of painful CGS in Lockdown due to the set in combination with the Striker class making you an almost untouchable Glass Cannon.
Demonic Spiders: Tier 2 and 3 Mecha Knights, all varieties. They have more hitpoints than nearly every other common mob, use combo attacks and an AoE charge attack like players do, run as fast as players while charging, shield-block projectiles, and inflict elemental status effects based on type. Their drawback is they periodically Shock themselves, except it's not really a drawback since if you're next to them when they do it you get Shocked too. This would be bad enough if they weren't often accompanied by Gremlin Menders and guaranteed to appear in groups in every single Arena.
In that case, go get a status bomb. Your life will be a lot easier. (Unless you get yourself into a dead end or a corner then you're screwed)
On the subject of Gremlin Menders, possibly the best example of a Demonic Spider is the Darkfang variety usually found on Tier 3 (though they might pop up on the last floor of Tier 2 if you're unlucky). As if their increased HP and speed wasn't bad enough, now they get 360 degree shields that will spawn as soon as they're near death; these have at least twice the health of the Mender and serve as an annoyingly large roadblock that you can't walk through. Oh, and they can still heal themselves and their allies while in the shield. Needless to say these guys put an end to a great many Arena runs. They were incredibly powerful in the Beta days due to healing themselves and thus constantly being driven back and forth from the critical damage point where they summoned the shield, to the point where the dev team had to put a cooldown on it. The general consensus nowadays is that if you can't kill a Darkfang Mender before it makes a second shield, then you have no business being on Tier 3.
Another example would be the Alpha Wolvers, before their nerf. They used to home after you when using 3-bite combo.
Rocket Puppies. Gun Puppies capable of doing hideous amounts of damage to you in one fire-inducing homing shot. Exactly how bad is it? The one and only Rocket Puppy in Firestorm Citadel has gained infamy for being difficult to sneak past.
Danger Missions often have their own exclusive enemies made specifically to make your trek through the clockworks absolute hell.
Apocrea Harvesters. Not only do they have a lot of health making fighting them futile, they can root you in place as they leech your health even if your shield is up, and the attack used to root you is difficult to get away from and can easily tear through a 4* shield, so it's less about taking your lumps for not being careful and more about taking it like a bitch if you don't have a teammate to prematurely free you.
Epileptic Trees: Are jellies victims of a horrible plague? All the Earth-shaped ruins and yet no plausible inhabitants...
Perhaps they were built by the Devilites, or whatever the zombies used to be before they died.
Fridge Brilliance: Aside from the overwhelmingly large amount of Slag Walkers and Trojans, there's a more obvious reason as to why Firestorm Citadel is a great place to grind Heat. The whole place is wreathed in flames.
Fridge Logic: The final mission in which you become a vanguard features several of the most prominent NPCs all there to congratulate you. Your intelligence provider, the guy who taught you all about the different types of damage and monsters, the Lieutenant who's been commanding the Knights all throughout the story... and also Bosco, the random NPC who was busy working on fixing a mecha-knight. You probably won't think much of it at first but if you take a moment to think about it, why is he there? (The answer, most likely, is because they wanted to show a mecha-knight applauding, but still...)
Goddamned Bats: Which enemies are considered this depends on who you ask. The more common targets of a player's hatred are Gun Puppies, Red Rovers, Wolvers, Gremlins, Tier 2 Retrodes, large groups of Devilites, and the Phantoms found in Graveyard floors.
A swarm of Greavers. They don't do particularly much damage, but they are able to home in on you when they wind up for an attack from almost an entire screen's worth of distance.
Red Rovers are a joke compared to Rocket Puppies, but Red Rovers were much worse than Rocket Puppies back when Fire was horribly broken and they used the bouncing flames they have at T3 in all tiers due to a bug.
Jellies get really annoying in groups. It isn't helped by the fact that they seem to resist flinching the deeper you go, causing mastery of shield canceling to be almost mandatory.
In some of the Danger Missions, Bombies. They explode when you come near them, they have moderately high health (enough that killing them before they reach you is unlikely), they explode when dealt lethal damage anyway, and they can revive when near Grim Totems or Deadnaughts.
Mortafires and Ghostmane Stalkers in Operation Crimson Hammer, the former because they constantly move around and can only be defeated without the use of fire pots by attacking them from behind, and the latter because they usually travel in groups, have high melee and ranged capability, can take a lot of damage, and can cloak to make a quick escape while also placing a deathmark to increase damage you take.
Ask any Tier 2 Recon in Lockdown about Pulsar line of handguns... There's much debate whatever it is balanced or not.
Goddamned Boss: Royal Jelly is pretty easy to take out (especially after they nerfed him), but he can easily turn into an energycostly nightmare.
The Roarmulus Twins count as well. While they seem to be a straightforwardly easy Puzzle Boss at first, once you factor in the endlessly respawning Scuttlebots and having to coordinate your teammates, the entire thing becomes harder.
Lord Vanaduke. He doesn't heal or have any particular fancy tricks, but there's a lot going on and he does lots of damage with all his attacks.
Godwin's Law: Not the actual Godwin's Law, but something similar exists on the forums that causes a lot of the discussions to move towards the question whether or not the energy system is a good idea. It's so common, in fact, that veterans can't take them seriously anymore.
Good Bad Bugs: There's a few, the most controversial being "Looping"- an exploit that lets you repeat levels (though it has since been patched).
Sometimes if a player is KO'd in front of Lord Vanaduke while he's in a corner he'll be incapable of getting out as long as that player remains in front of him.
During the Dark Harvest Halloween event in 2012, the developers tried to prevent players from doing the relevant prestige mission more than once per day, but through joining other parties that had just started the mission they were able to do it repeatedly. Then came the Winterfest event and it appears they gave in and let players do the relevant mission as much as they wanted without any sort of trouble.
I Am Not Shazam: It's the ROYAL JELLY. It lives in the Royal Jelly Palace. The items are called Royal Jelly Mail, Royal Jelly Helm, Royal Jelly Shield, and Royal Jelly Band. There is no "king" anywhere◊.
Most Annoying Sound: Enemies that can heal others do so with a cartoonish car horn sound effect. If there are multiple Mend-capable enemies in a single area, it gets old rather quickly. This may lead to possible Fridge Brilliance if you take the sound effect as a cue on who to kill first.
That One Achievement: "Master Miner" (deposit 10000 minerals into a gate) and "Dauntless Delver" (go from 0-29 without dying). The latter is made easier if you have a buddy to take the hits for you and if you pick simpler levels, but still requires a considerable chunk of time and Crystal Energy (or an elevator pass). The former is simply tedious; assuming best conditions (4 maximum sized minerals on every level, and 4 players to carry them all up) which lets face it, isn't going to happen (especially since some levels in missions don't even have any crystals), it still takes a bare minimum of 625 levels to get. Even highly dedicated players require several months to get Master Miner.
That One Attack: The Roarmulus Twins possess a laser attack in their final phase. Unlike all other attacks in the game, the beam is a constant attack. And it can inflict Shock, a status that removes the player's Mercy Invincibility. These two factors combine to create an attack capable of a Total Party Kill in a split second.
The Graveyards. Not only are they overrun with zombies, but you also have to fight ghosts of Spiral Knights, that regenerate mere minutes after being defeated and then chase after you again. And they don't even drop loot. But on the other hand, you have a level inhabited with a single type of very slow enemies. Once you can reliably kill Phantoms, you can clear the level using only one weapon, leaving your second weapon slot free for whatever low-level weapon you want to heat up. It's still tedious, though.
The danger rooms (though optional) and any areas can count as those as well.
Candlestick Keep also tends to draw a lot of ire. Throughout the level, you have to stay within range of the candles' light, or else you'll be chased by the Grimalkins, whose attacks deal considerable damage and ignore your shield. What's more, some of the candles are only temporary, and will have to be relighted with fireballs. Combined with the fact that you'll often be dodging attacks from Howlitzers and Kats, some people choose to just wait for these levels to pass by.
For elemental stratums, enemy types can circulate unpredictably. Those who hate Wolvers, for example, may find themselves forced upon going to a Wolver Den. Some people may prefer to stick with the Graveyards.
The Ironclaw Munitions Factory can be utter hell if you don't know what you're doing. The whole area is basically one long gauntlet of you dodging super powerful missiles and activating many, many switches, all while Quicksilvers, Scuttlebots, Retrodes and Mecha Knights wail on you. Oh, and due to all the stuff being thrown at you at once, the level is notoriously buggy and laggy. And at the end, you face the Roarmulus Twins. Going solo is not advisable.
Now we have the Shard Bombs.... The developers reasoned that the change was made because they didn't intend to make players use Shard Bombs for sniping things like Gun Puppies.
The removal of Winmillion line's first attack dash to some. Turning them from a sword that let users happily dash at target from yards away without the risk of getting attacked first to more of a backup weapon for bombers, being able to shoot out bullets instead of bringing in a gun with no melee capabilities.
Pretty much everything about the July 30 update outside of battle sprites, especially things like removing the ability to share health and heat with your friends. It didn't help that Three Rings never really explained that the reason for most of the changes was to starve the users who were using large numbers of alts to make far more money than intended.